Piezo-electric oscillator



Nbv.25,l930. L783J30 PIEZO ELECTRIC OSCILLATOR Filed June 24:: 1925 IN VEN TOR BY W Patented Nov. 25, 1930 warren stares Parent orator;

RUSfiELLfE. 0H1 0F NEW YORK, N. Y., ASSIGNOR TO AMERICAN TELEPHONE AND TELE- GRAPH \IOMPAIQ'Y, it CORPGRATIOLI OF N PIEZO-ELECTEIC CSCILLATOR Application filed June 24, 1926.

This invention relates to oscillation generators, and particularly to oscillation generators employing piezo-electric devices for varying the frequency of the oscillation generators.

Piezo-electric devices have heretofore been employed to control the frequency of oscillations generated by space discharge tube oscillators. These piezo-electric devices comprised piezo-electriccrystals each vibrating between two conductive plates. These piezoelectric crystals were usually cut from a substance, such as quartz, so that the dimension in the'direction of the axis of vibration of the crystal was small as compared to the other dimensions.

It has been found that in a space discharge tube oscillator employing a piezo-electric crystal, which vibrates between two conductive plates, for controlling the frequency of the space discharge tube oscillator, and in which one of the conductive plates is in contact with one surface of the piezo-electric crystal, and in which there is an air gap between another surface of the piezo-electric crystal and a second conductive plate, the frequency of the alternating current generated by such a space discharge tube oscillator can be controlled by varying the size of the air gap between the piezo-electric crystal and the latter conductive plate. In experimenting with such a crystal controlled space discharge tube oscillator, it has been found that as the size of the air gap changed, the frequency of vibration of the piezo-electric crystal also changed, causing a corresponding change in the frequency of the alternating currentgenerated by a space discharge tube oscillator. As the size of the air gap increased, the frequency of vibration of the piezo-electric crystal also increased, and vice versa. The change in the frequency of vibration of the piezo-electric crystalappears to vary in accordance with the width'of the air gap between the surface-of the crystal and the corresponding conductive plate.

Yet, when the size of the airgap becomes too great, the piezoelectric crystal may cease vibrating by reason of the fact that the dielectrio-properties of the air are such that the Serial No. 118,346.

potential applied between the conductive plates is insufficient to set up the necessary potential across the active faces of the piezoelectric crystal in order to start and maintain it in vibration.

In this invention there will be shown one form of embodiment of the invention in which a space discharge tube oscillator is employed for generating an alternating current wave, and in which a piezo-electric device is associated therewith for controlling and varying the frequency of the alternating current generated by the space discharge tube oscillator. The frequency of vibration of the piezoelectric crystal is changed by varying the size of the air gap between the piezo-electric crystal and one conductive plate. Yet, it will be understood that it is within the scope of this invention to associate any variable impedance with the piezoelectric crystal so that as the impedance varies, a change will be made in the frequency of vibration of the piezo-electric crystal.

This invention has one application in a space discharge tube oscillator, the frequency of which must be brought into synchronisni with those frequencies generated and transmitted by other transmitting stations. In accordance with this invention, by means of a very fine adjustment of the air gap between the piezo-electric crystal andone conductive plate, the frequency of vibration of the piezoelectric crystal may be very carefully calibrated.

It is therefore one of the objects of this invention to provide a. piezoelectric device comprising a piezoelectric crystal which may freely vibrate between two conductive plates, so that the frequency of vibration of the piezoelectric crystal may be changed as the size of the air gap between the piezo-eleotric crystal and its conductive plates changes.

It is another object of this invent-ion to so relate a piezo-electric device with an electrical oscillator and a. harmonic producer for producing any harmonic of the frequency generated by the electrical oscillator, that the frequency of vibration may be controlled and varied by suitable adjustment of the vibraice tory characteristic of the piezo-electric device.

This invention will be better understood when read in connection with the accompanying drawing showing only one embodiment of the invention, in which the frequency of an electrical oscillator, which is coupled to either an amplifier or a harmonic producer, is controlled and varied by means of a piezo-electric device.

In the particular embodiment of this invention, shown herein merely for the purpose of illustration, a space discharge tube oscillator is shown which includes a circuit arranged to set up oscillations of any frequency within the limits of a predetermined band. The reference character is the space dis charge tube of this oscillator which works on the thermionic principle, including a cathode, an anode and an input or grid control electrode. The grid electrode is connected to a conductiveplate L on which is placed a piezo-electric crystal PE. Another conductiveplate L is connected to the cathode of the space dischargetube V through an adjustable thumb screw S and an electrical terminal T. The conductive plate L is rigidly fixed to the adjustable thumb screw S so that as the thumb screw is moved inwardly or outwardly, a corresponding change will take place in the distance between the upper surface of the piezoelectric crystal PE and the conductive plate L Obviously, the width of the dielectric, such as air, between the'conductive plate L and the surface of the piezoelectric crystal PE may be changed by manipulation of the thumb screw S. The piezoelectric device is located in a. box or other container which is made of hard rubber, bakelite or other dielectric material. A graduated scale G is so located with respect to the thumb screw S that as the thumb screw S moves along the threaded portion of the box conteining the piezoelectric crystal, the displacement of the thumb screw may be readily determined. It is obvious that it is within the scope of this invention to provide any means whatsoever for rendering a. fine adjustment of the space between one or two active surfacesof a piezo-electric crystal and the corresponding conductive plates adjacent thereto, so that the frequency characteristic of the piezc-electric crystal may be carefully calibrated.

The cathode of the space discharge tube V is heated to an electron emitting temperature by the flow of current from a battery B A battery B supplies a space current to the anode of the space discharge tube V through the upper portion of the winding V1 A resistanceR is connected between the grid elec trode and the cathode of the space discharge tube Vgin order to maintain the grid electrode at a suitable negative potential with respect to the cathode. A condenser C shown 'within the tube.

nected between the grid electrode and cathin dotted lines, represents the inherent capacity between the grid electrode and the anode including the capacityof the leads connected thereto. It is this capacity between the grid electrode and the anode which produces a reaction of the plate circuit on the grid circuit in order to produce sustained oscillations.

Another space dischar e tube V is coupled to the oscillation generator. The grid electrode of this space discharge tube is connected to the winding l/V through a condenser C which is of low impedance so that alternating current waves may freely pass therethrough. Yet, this condenser is arranged so as to prevent the potential of the battery B from bein applied to the grid electrode of the space discharge tube V The cathode of the space discharge tube V is also heated to an electron emitting temperature by the flow of current from the battery B A battery B supplies the space current for the anode of the space discharge tube V through the lower portion over the winding W The anode of space discharge tube V2 ,is connected to the lower terminal of the winding 7 through a condenser G which is in series with a portion of the winding 7 This condenser C is the balancing condenser for the space discharge tube V a balancing the inherent capacity existing be tween the plate and grid electrodes of the space discharge tube V including the ca;

pacity existing between the leads connected thereto. A current flows through the condenser C and that portion of the winding N in series therewith, which is equal in .magnitude and opposite in phase to that flowing within the tube by virtue of the inherent capacity existing between the plate and grid electrodes thereof. That portion of the winding W which is in series with the condenser C is employed for adjusting the phase of the current flowing through the condenser C so that this current is in phase opposition to that flowing inherently A resistance is ode of the space discharge tube V in order to maintain the grid electrode at a suitable negativepotential with respect to the cathode.

A tuned circuit is associated with the output circuit of the spacedischarge tube V This tuned circuit comprises the winding W2 and a variable condenser C in shunt therewith, this tuned circuit transmitting to an antenna A'current of a frequency which is characteristic of the tuned circuit- An arm moves over the winding W so that the cou- 'pling between the antenna and the tuned circuit may be regulated.

The piezo-electric crystal PE is so ar-' ranged that it mayv freely vibrate between the conductive plates L and L asmall air COl1- It'll gap existing between the upper surface of the piezoelectric crystal PE and the conductive plate L The crystal is preferably disk-shaped, cut from a substance such as quartz, alon one of the axes of symmetry. It is a well-known property of a crystal so cut that when subjected to a physical stress from the sides of the crystal, plus and minus charges appear on the faces of the crystal, and conversely, when the crystal is subjected to the plus and minus charges of an electrostatic field, a physical deformation occurs along the axis of the crystal. The signs of the charges produced in the first case depend upon the character of the stress; that is, whether the stress is tension or compression, whereas in the second case the physical deformation due to the action of the electrostatic field depends upon the direction of that field. If the physical stress changes from tension to compression, or vice versa, the signs of the electric charges on the faces of the crystal are reversed, whereas, if the direction of the electrostatic field is alternately reversed, the crystal undergoes contraction and elongation alternately in the direction of the axis. In other words, the crystal vibrates in an electrostatic field in which the charge is alternately reversed.

The piezo-electric device employed herein varies and controls the frequency of oscillations generated by the space discharge tube V If the frequency of the piezo-electric device changes, there will be a corresponding change in the frequency of oscillations flowing in the space discharge tube V Moreover, if the space discharge tube V is employed as aharmonic producer or frequency multiplier, the frequency of the tube V will also be varied and controlled by the piezoclcctric device. In the latter case, the tuned circuit, which comprises the winding W and the variable condenser C would determine whether or not the space discharge tube V is employed as a harmonic producer or frequency multiplier. If the spacedischarge tube V is to be employed as a harmonic producer or frequency multiplier, the tuned circuit might be adjusted so that its frequency would be some multiple of the frequency of oscillations generated by the space discharge tube V On the other hand, if the space discharge tube is employed as an amplifier to amplify the oscillations generated by the space discharge tube V the frequency characteristic of the tuned circuit would have to be substantially equal to that generated by the space discharge tube V Nevertheless, the frequency transmitted by the combination of the space discharge tubes V and V would be controlled and varied by means of the piezo-electric device.

The resistances R and R associated with the space discharge tubes V and V respectively, are employed herein instead of one or possibly two local batteries to maintain the corresponding grid electrodes at appropriate negative potentials with respect to the corresponding cathodes. This means of maintaining the grid negative operates only when there is a convection current flowing between the cathode and the grid electrode, i. e., the resistance, tends to maintain the grid electrode negative with respect to the cathode during that part of the alternating current cycie in which there is apositive charge 011 the grid'electrode. When the grid electrode becomes positive, electrons are attracted thereto so that there is a flow of current from the grid electrode to the cathode through the resistance therebetween. During that part of the alternating current cycle in which the grid electrode is negative with respect to the cathode, there is no flow of electrons from the cathode to the grid electrode. Accordingly, a rectified current flows through each resistance, thereby lowering the potential of the grid electrode with respect to the cathode.

The term piezoelectric, as employed herein, refers to that property of a body or substance which produces a mechanical force in response to an electromotive force, and vice versa.

While this invention has been shown in one particular embodimentmerely for the purpose of illustration, it is obvious that the general principles of this invention may be applied to other and widely varied or anizations without departing from the spirit of the invention or the scope of the appended claims.

hat is claimed is:

1. The combination of a piezo-electric crystal, two conductive plates between which the piezo electric crystal is placed so that it may freely vibrate therebetween, means for manually adjusting the air gap between one conductive plate and the corresponding surface of the piezoelectric crystal, and means for indicating the distance between the piezoelectric crystal and the latter conductive plate.

2. The combination of a pieZo-electric device, a holder for said device including means for varying the airgap between the piezoelectric crystal and one of its conductive electrodes, and a graduated scale supported by the holder for indicating the vibratory period corresponding to the airgap between the piezoelectric crystal and its conductive elec trodes.

3. A mounting for housing a piezo-electric crystal including a pair of conductive electrodes of adjustable spacing providing an air gap for the crystal, and means fastened to the mounting for indicating the spacing of said electrodes.

4. The combination of a piezo-electric crystal, two plates adjacent thereto, means for varying the spacing between one of said Cir plates and said crystal in order to produce a variation in the vibratory period of said crystal, and means for indicating the tween said plates. V

5. The combination of a piezoelectric structure provided With an air gap, screwthreaded means for manually adjusting the Width of the gap to a difierent value, and means for indicating the vibratory period corresponding to the Width of said gap.

, 6. The combination of a piezo-electric crystal, two conductive electrodes one of which is spaced from the crystal, and means for indicating the size of the spacing.

7. The combination 0]": apieZo-electric crystal, two electrodes adjacent to said crystal, means for varying the spacing of one of said electrodes With respect to the corresponding distance be surface of said crystal, and means for indi- V eating the size of said spacing.

S. A mounting or holder for a piezo-electrio crystal having tWo electrodes, one electrode being permanently fastened to said mounting or holder and the other movable to provide an air gap for the crystal, and a graduatedscale for indicating the spacing betweensaid electrodes. r

Intestimony whereof, I have signed my name to this specification this 23rd day of June, 1926. v V p e v RUSSELL S. OHL; 

